N4729D, deregistered, just flew over my house. I saw where it had crashed years earlier; is it legal to fly this plane?
It isn't legal to fly an aircraft without a valid registration, however if the owner has applied for a new registration then they can fly the aircraft using the application as a temporary registration.
So it's possible that the owner has repaired the aircraft, re-applied for registration, and is now flying the aircraft using the application as temporary authorization.
The general requirement to have a valid registration certificate on board is in 14 CFR 91.203(a) says:
Except as provided in § 91.715, no person may operate a civil aircraft unless it has within it the following:
(2) An effective U.S. registration certificate issued to its owner or, for operation within the United States, the second copy of the Aircraft registration Application as provided for in § 47.31(c), a Certificate of Aircraft registration as provided in part 48, or a registration certification issued under the laws of a foreign country.
According to the FAA's N-number registry, N4729D is indeed deregistered. That doesn't mean it's flying illegally, though. 91.203 mentions 47.31(c), which essentially says that if you've applied for a registration then you can use a copy of the application as a temporary authorization:
After compliance with paragraph (a) of this section, the applicant for registration of an aircraft last previously registered in the United States must carry the second copy of the Aircraft Registration Application in the aircraft as temporary authority to operate without registration.
This question might be helpful too.